Conventional self-ventilated bi-directional traction units for rail vehicles include a traction motor shaft, which can rotate in both direction to drive a vehicle in both a forward and backward direction, and a fan directly mounted on the motor shaft to cool the motor. To obtain the same cooling performance irrespective of the direction of travel, the fan usually has a so-called symmetric design, i.e. with symmetric blades, and the efficiency of the fan is not optimised, which means that the dimension, energy consumption and generated noise are higher than desirable. Alternatively, the fan may be equipped with controllable pitch blades. But such equipment is particularly costly to manufacture and maintain.
Departing from the usual symmetric fan arrangement, a self-ventilated bi-directional traction drive unit known from DE19842473 comprises a bi-directional motor, an asymmetric fan for ventilating the motor, and a mechanical transmission device between the motor and the fan. The mechanical transmission is operative to drive the fan rotor in one and the same direction and with a constant gear ratio 1:1 irrespective of the direction of rotation of the motor shaft. The mechanical transmission includes a planetary gear train comprising a ring gear, a sun gear and a set of planet gears supported on at least one planet carrier. Free wheels are provided between the planet gears and the planet carrier to lock the planet gear in one direction of rotation. This arrangement provides interesting results in terms of direction of rotation of the fan and speed ratio. However, the offset position of the free wheels with respect to the main rotation axis of the fan and of the motor shaft may result in rapid and excessive wear and deterioration of the free wheel in use. Moreover, the gear ratio 1:1 is inherent to this structure and cannot be changed without adding a reduction gear. Hence, the gear ratio cannot be optimised to meet the cooling and noise emission requirements.
A differential motion gear system to control the speed ratio by means of the change of input direction is disclosed in U.S. Pat. No. 5,607,369. This system causes the changing of the output speed ratio by changing the revolving direction of the input shaft of the differential motion gear system. In one embodiment, the differential motion gear system comprises: an input shaft rotatable in opposite directions; an output shaft; a first one-way drive mechanism connected between the input shaft and the output shaft; a sun gear connected to the input shaft; a differential motion gear engaged with the sun gear; a ring gear engaged with the differential motion gear; a stationary case; and means attaching the ring gear to the stationary case so as to prevent relative rotation between the ring gear and the stationary case. As a result, the speed ratio is −T1/T2 in one direction and 1+T1/T2 in the other direction, where T1 is the number of teeth of the sun gear and T2 the number of teeth of the ring gear. While the output shaft rotates in the same direction irrespective of the direction of rotation of the input shaft, the significant difference in the speed ratios, which is the main purpose of this system, makes it unsuitable for self-ventilated drive unit.
Prior document DE10 2007 020345 discloses a self-ventilated bi-directional traction drive unit comprising a bi-directional motor, a fan for ventilating the motor, and a mechanical transmission between a motor shaft of the motor and a fan rotor of the fan, wherein the mechanical transmission is operative to drive the fan rotor in a preferred fan direction with a first constant gear ratio when the motor shaft rotates in a first traction direction and to drive the fan rotor in the preferred fan direction with a second constant gear ratio when the motor shaft rotates in a second traction direction opposite to the first traction direction, wherein the mechanical transmission includes a planetary gear train comprising a sun gear, a ring gear and a set of one or more planet gears supported on at least one planet carrier and further includes free wheels, wherein one of the free wheels operates between the ring gear and the fan rotor to block the rotation of the fan rotor with respect to the ring gear in the preferred fan direction.
EP 2 444 312 discloses a speed changing apparatus, which uses clockwise and anti-clockwise rotation of a motor couple with a planet gear set to achieve the purpose of a gear shift, i.e. to provide different output gear ratios in the same output rotation direction. To this end, the speed changing apparatus comprises a bi-directional drive unit comprising a bi-directional motor, an output ring sleeve, and a mechanical transmission between a motor shaft of the motor and the output ring sleeve, wherein the mechanical transmission is operative to drive the output ring sleeve in a preferred direction with a first constant gear ratio when the motor shaft rotates in a first traction direction and to drive the output ring sleeve in the preferred direction with a second constant gear ratio when the motor shaft rotates in a second traction direction opposite to the first traction direction, wherein the mechanical transmission includes a planetary gear train comprising a sun gear, a ring gear and a set of one or more planet gears supported on at least one planet carrier and further includes free wheels. A first of the free wheels operates between the planet carrier and the output ring sleeve to block the rotation of the output ring sleeve with respect to the planet carrier in a direction opposite to the preferred direction and a second of the free wheels operates between the ring gear and the output ring sleeve to block the rotation of the output ring sleeve with respect to the ring gear in the preferred direction.